1. Field of the Present Invention
The present invention relates generally to the installation of stringlines for guiding construction equipment, and, in particular, to the automated installation of stringlines, for guiding slip form paving machines, using 3D control data.
2. Background
In the construction industry, a longstanding issue has been how to accurately determine, on the construction site, the desired location for a building, road or other construction project as specified in plans developed by an architect, engineer, or the like. Most commonly, surveying techniques, supplemented in recent decades by advances in surveying technology, have been used to pinpoint and mark precise locations on a construction site, thereby guiding construction workers as they work.
Unfortunately, during construction, the locations marked by the surveyors may be affected by the construction process itself. For example, stakes that are laid out by surveyors to mark the edges of a planned road may be moved, covered or destroyed by earth-moving equipment as excavation, fill or the like is carried out. As a result, construction must often be halted temporarily while surveyors reestablish the construction locations, and then the earth-moving process is continued.
More recently, advances in global positioning system (“GPS”) technology have begun to find applicability in the construction industry. Perhaps most obviously, GPS technology is now widely used by surveyors in finding locations because it permits actual physical locations to be determined with accuracy to the hundredth of a foot. Because the plans for most construction projects today are developed via computer, such techniques are particularly useful because the plans can be coordinated with the GPS data, thereby providing precise guidance during the surveying process.
In addition, however, GPS has begun to be used to guide the operation of construction equipment during the construction process itself. In fact, the use of so-called three-dimensional (“3D”) controls to direct the operation of construction equipment is becoming increasingly common, particularly with regard to earthmoving equipment. A typical implementation of a 3D control system in such a context involves the use of one or more fixed base stations, located in and around the construction site, coupled with a mobile unit disposed on the individual construction equipment that is to be controlled via the system. As described below, the type of control system used may vary, but in each case, the exact position of each base station may be established by conventional surveying means, supplemented by the use of GPS technology.
In one type of 3D control system, the mobile unit is also a GPS unit, and thus the position of the mobile unit, and indirectly, the construction equipment on which it is carried, may be determined with some accuracy using only the mobile unit. The position information provided by the mobile GPS unit by itself is of only limited accuracy. However, in this arrangement, the GPS data developed by the mobile unit may be supplemented and adjusted, as appropriate, using additional GPS data from the fixed base stations, the position of each of which is known with great accuracy. This, in turn, provides highly accurate information about the exact position of the mobile unit, and indirectly, the construction equipment. Such a system is sometimes referred to as a real time kinematic (“RTK”) GPS system.
More commonly, however, the base station is a robotic laser-based tracking station, sometimes called a “total station,” and the mobile unit includes a prism, wherein the robotic tracking station produces one or more lasers and directs them toward the construction equipment, and more particularly, toward the prism, which is mounted in a prominent location on the construction equipment to maximize its ability to receive the laser. In this type of 3D control system, the laser is used to determine the position of the prism relative to the base station by calculating distance and angle. Because the position of the base station is known, the position of the prism, and indirectly the position of the construction equipment, may be established using the combination of the GPS information developed by the base station and the relative positional information provided using the laser and prism.
For a variety of reasons, however, it is very difficult to use these systems to control equipment such as slip form paving machinery, an example of which is shown in FIG. 1. For one thing, regardless of which type of system is used to determine it, position by itself is not sufficient to control the operation of the equipment. For example, steering a construction machine further requires knowledge of the machine's orientation in two-dimensional space. Conventionally, the machine's orientation is determined indirectly as being closely related to the machine's direction of travel. Currently, determining a machine's travel direction involves comparing the machine's current location, determined via one of the previously-described systems, to its previous location. The vector defined by those two points approximately defines the machine's current direction of travel.
Unfortunately, this approach includes a number of inherent inaccuracies, particularly for curbing machines and other slip form paving machines. First, this approach is dependent upon sufficient movement by the machine in a straightforward direction. The approach cannot work at all if the machine is not moving, because direction of travel cannot be determined in this way if the current location and the previous location are the same. Further, the approach may be highly inaccurate if the current location and the previous location are particularly close to each other, which may happen if the machine is operating in a confined area or is of a type that can spin in place or turn with a very tight turning radius.
Another inaccuracy stems from the fact that machine orientation is not exactly equivalent to direction of travel. For example, it is impossible to determine precisely whether the path traveled by the machine from its previous location to its current location followed a straight line or a curved one. Because the orientation of the machine at the current location will be different if the machine followed a straight line to get there than if it followed a curved one, this uncertainty translates to corresponding uncertainty in the orientation of the machine.
Yet another inaccuracy stems from the use of positional data for only a single point (the point at which the mobile unit is positioned on the machine) to represent the position of the entire machine. In fact, most machines are several meters wide, several meters long and at least a couple of meters high. Because GPS (coupled with one of the systems described above) may be used to determine location to accuracies of considerably less than a meter, the positional data thus determined is accurate only for a small part of the machine, i.e., the exact location of the mobile unit on the machine. The position or location of other parts of the machine, such as the machine's operational components, may be determined only by combining information about the relative disposition of the mobile unit on the machine with knowledge of the geometry of the machine. For machines whose typical use involves travel only in a linear direction, and deviations from such travel occur only infrequently, this approximation may be acceptable. However, for other types of machines that turn regularly, or whose operational components move or are adjusted dramatically relative to the rest of the machine, the error induced between the fixed position of the mobile unit and the position or orientation of the operational components can become dramatic, thus rendering the use of such a system unsuitable for controlling certain types of machines.
The significance of this problem increases in relation to the degree of independence with which the operational components of the machine move relative to the movement of the machine itself. For example, in a curbing machine, the slip forming equipment mounted on the machine is typically adapted to form curbs having very short radiuses of curvature while the machine itself moves forward or stops altogether. When this type of curb is being formed, the movement of the operational components is thus very different from the movement of the machine itself. Conventional systems are ill-equipped to address this issue.
Paving and curbing equipment further require the attitude of the machine side-to-side (generally referred to as “cross slope”) and the attitude of the machine front-to-back (generally referred to as “long slope”) to be accurately controlled in order to maintain the proper three-dimensional form (side-to-side and front-to-back) of the pavement or curbing being formed. Traditionally, the machine location, direction, and long slope is referenced from a stringline that is placed ahead of time to guide the location of the slip-forming equipment on the machine, while cross slope is monitored by a cross slope sensor. If a 3D control system of one of the types described hereinabove is applied to such equipment, the only information continuously established with regard to the machine is the location of the single mobile unit (most often, a prism); all other information must be extrapolated, with varying degrees of accuracy, or must be developed using other means. For example, the determination of long slope requires an additional sensor over and above the cross slope sensor. Such a sensor is not usually provided on stringline-controlled machines, and thus represents an additional complication in the application of conventional 3D control systems to, for example, paving and curbing machines.
Notwithstanding the foregoing, advances have been made recently in this technology area, including early development of an automated paving machine, as described in co-pending and commonly-assigned U.S. Patent Application No. 60/910,251 (“the '251 patent application”), the entirety of which is incorporated herein by reference, that utilizes a 3D control system for a paving machine. Such advances may address some of the aforementioned problems, but no commercial product has yet to be provided. Moreover, such a 3D control system may be difficult to use while maintaining a high level of accuracy. Furthermore, it might be cost-prohibitive to equip all construction machines on a job site with 3D control systems. For these reasons and others, it may continue to be useful to install and use stringlines for guidance of conventional paving machines, and it would be foolish to ignore the traditional importance of the stringline in establishing, indirectly, the location of other features as well. Conventionally, the stringline is one of the first construction elements put in place on a construction site. Other construction elements are either placed based directly on the stringline or are placed based on the paving structure that is built by the paving machine.
Nonetheless, the benefits of GPS and laser-based 3D systems are apparent. Thus, until accurate 3D control systems for stringline-guided paving machines are widely and economically available, a need exists for a system that utilizes at least some of the benefits of GPS and laser-based 3D systems for aid in pointing to and/or ultimately controlling the placement of physical references such as stakes or stringlines.